Patent GB-1070999-A (GB)
Title: Improvements in or relating to circuit arrangements for producing a switching signal in colour television apparatus
IPC: H04N9/44
Filed: 1965-09-17
Abstract: 1,070,999. Colour television; transistor pulse circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Sept. 17, 1965 [Sept. 19, 1964], No. 39757/65. Headings H3T and H4F. The invention relates to a receiver for receiving a PAL-signal and an NTSC-signal and also to a circuit arrangement for converting a PAL-signal into an NTSC-signal and vice-versa in which a dissymetrical switching signal, having one polarity during one scan period plus two fly-back periods and the other polarity during the scan period of the subsequent line, is utilized for switching the phase of at least one colour component of the television signal or a sub-carrier wave signal derived from the burst signal from line to line, characterized by the use of a relaxation generator, described below, for supplying the dissymmetrical switching signal. Because the switching signal has one polarity for the duration of one scan period plus two fly-back periods the burst experiences no influence of the switching. A first embodiment, Fig. 1, relates to a PAL- receiver in which a signal according to the PAL-system is applied at input terminal 1. The luminance signal Y is applied via amplifier 2 to the tube 3 and the modulated colour subcarrier is applied via colour amplifier 4 and synchronous demodulators 5 and 6, which supply the red and blue colour difference signals, to the tube 3, the green colour difference signal being derived in device 7. The signal for the automatic contrast control for the colour amplifier 4 and the control signal for the local sub-carrier oscillator 16 are derived from the (B-Y) and (R-Y) demodulator respectively by means of comparison stages 9 and 13 gated by line fly-back pulses 10 and reactance circuit 15. A transformer 17 which comprises a primary winding 18 and two secondaries 19 and 20 is arranged at the output of oscillator 16, and by means of switching signals 32 and 33 supplied by relaxation generator 23 the diodes 24 and 29 are switched so that the sub-carrier wave signal may pass to demodulators 5 and 6 via phase shifting networks 25 and 30 as shown. The Specification explains the operation of this circuit arrangement with reference to a vector diagram, Fig. 2, not shown, and also why during a fly-back period, namely the period that a burst signal is transmitted, diode 29 is arranged never to be in the conducting condition. This is arranged by making the switching signals 32 and 33 dissymmetrical, i.e. the time T 1  that the switching signal 32 opens the diode 24 is chosen to be longer than the time T 2 , being the time that the switching signal 33 opens the diode 29. Specifically the switching signal has one polarity during one scan period plus two fly-back periods, one of the associated line and one of the subsequent line, and has the other polarity during the scan period of the subsequent line, Fig. 3, not shown. The switching generator which produces the switching signal from line fly-back pulses 34 comprises a monostable multivibrator circuit 23, Fig. 5, in which the line fly-back pulses 34 are applied through line 35 and the output switching signals 32 and 33 are derived from transistors 53 and 52 via capacitors 65 and 66 respectively, the pulse synchronization signal transmitted with the PAL-signal being supplied at input terminal 36 to establish the correct phrase of the switching signals. Capacitor 64 and resistor 62 differentiate the fly-back pulses 34 and in operation with transistor 52 initially conducting and transistor 53 non-conducting, i.e. the stable condition, the arrival of the differentiated leading edge of the first fly-back pulse causes the voltage across capacitor 63 to exceed the cut-off voltage of transistors 52 and drives the circuit into an unstable condition with transistor 53 conducting and transistor 52 non-conducting. The arrival of the differentiated trailing edge of the second pulse turns on transistor 52 and thereby reintroduces the stable condition. A second embodiment, Fig. 7, relates to a receiver which is suitable for receiving a PAL- signal and an NTSC signal. The signal derived from the colour amplifier 4 is fed through four different paths including phase inverter 70, one line period delay circuit 73, adding stages 71 and 74 and a gate circuit 77 which is released only during the line flyback period when a PAL-signal is received but is fully released when an NTSC signal is received. When a PAL-signal is received the output from adding stage 71 comprises the I colour component only and this is fed through an impedance matching circuit 79, combined with the burst signals from gate 77 and applied to diodes 81 and 82 via capacitor 80. When the signal +I appears diode 81 is released by the dissymmetrical signal 32 from generator 23, and when the signal -I occurs the diode 82 conducts the signal to transformer 83 which inverts it in phase and thus always a signal +I appears at the primary of transformer 84. The output of adding stage 74 comprises the Q colour component only and this is fed via a 90 degrees phase shifting network 86 to the centre tapping 85 and by synchronously demodulating the resultant signal at the ends of the secondary of transformer 84 the red and blue colour difference signals are derived. When an NTSC- signal is received gate 77 is fully opened and diode 81 is arranged to be always released by omitting the switching signal 32. Chromatic aberration will occur due to phase errors when an NTSC-signal is received but when a PAL- signal is received there is no chromatic aberration. A further embodiment, Fig. 4, not shown, which is described in detail in Specification 1,070,998 relates to a circuit arrangement utilizing the dissymmetrical switching signal for converting a PAL-signal into a NTSC-signal and vice-versa.